If you open up the big, heavy outer casing of the attenuator, you will discover the internal workings are actually very compact and lightweight. I suppose the outer casing must provide some type of shielding?!
Anyway, I'm thinking it might be wise to just mount the internal board inside your Boombox intercepting the phono circuit and the external inputs. The attenuator doesn't need to be powered so it should be any easy mod.